By increasing a support surface, side extension arms are able to increase the stability of the parked vehicle, whereby, for example, in the case of a vehicle fitted with a crane, the steadiness of the vehicle is increased. While the vehicle is moving, it is necessary to ensure that the side extension arms remain in their fully retracted transport position and do not by mistake move into a partially or completely extended position. In this regard, a regulating standard requires that transport securing arrangements for side extension arms of this kind include a locking device and an additional independent securing hook.